1. Field of the Invention
The present invention relates to an immersion microscope objective lens, and more particularly relates to an immersion, high magnification, apochromat microscope objective lens.
2. Related Background Art
In immersion microscope objective lenses having high magnifications, in order to correct curvature of field, a plano-convex lens having an refractive index lower than that of the frontmost lens (i.e. the lens closest to the object) is inlaid in-the frontmost lens with the planer surface facing the object side to give to the cemented surface therebetween a negative refractive power utilizing the refractive index difference between these lenses so as to reduce the Petzval sum. In addition, a so-called Gauss type lens configuration in which concave surfaces opposed to each other are disposed at a position at which the diameter of the light flux becomes small to reduce the Petzval sum by their strong negative refractive power is also known. The combination of these structures is well known as a structure that can effectively reduce curvature of field.
For example, high magnification, apochromat microscope objective lenses disclosed in Japanese Patent Application Laid-Open Nos. 7-230038, 2000-35541 and 2002-148519 have been known as examples of immersion microscope objective lenses having the above-described structure. In the immersion microscope objective lenses disclosed in these patent documents, the surface of the inlaid lens (i.e. a small diameter lens inlaid in the frontmost lens) that is in contact with the immersion liquid is designed as a planar surface in order to facilitate operations of removing air bubbles and foreign matters etc. in the immersion liquid or in view of ease of processing of the lens itself. In the case that the immersion liquid is oil, the refractive index of the immersion liquid and the refractive index of the inlaid lens can be made substantially equal to each other. (The refractive indices of the oil and the glass are approximately 1.5.) Therefore, even when the surface of the inlaid lens that is in contact with the immersion liquid (which surface will be referred to as the first lens surface) is a planer surface and the incidence angle of incident light is larger than 60°, aberrations generated at that surface are insignificant and errors in the dimension of the aforementioned first lens surface and the thickness of the center have little influence on aberrations.
However, it is preferable for some kinds of specimens to be stored and observed in a state immersed in water. For such specimens it is preferable to use water as the immersion liquid of the immersion microscope objective lens. In the case of the water immersion liquid, its refractive index is as low as 1.33, which is smaller than that of the lens (made of a glass). Thus, there arises the problem that spherical aberrations occur at the interface due to the refractive index difference with the lens. In particular, in designing a lens having a large field of view with a numerical aperture (NA) more than 1.2 while ensuring a minimum necessary working distance, significant high order spherical aberration occurs, and it is difficult to correct the aberration by the succeeding lens groups. In practice, it has been very difficult to correct the spherical aberrations if the difference in the refractive index between the immersion liquid and the lens (or the water and the glass) is larger than 0.1, the aforementioned first lens surface is planar and the numerical aperture of is larger than 1.2.